The invention concerns a process for the production of surface coatings as well as the use of a surface coating to increase the surface tension of objects.
The coating of surfaces of solid objects is a widely used means of specifically changing the surface properties of objects. Here reference is only made to the countless methods of corrosion protection for everyday objects made of base metals or the painting of objects with the aim of changing their optical appearance.
Coating is usually understood as a manufacturing process for applying a firmly adhering layer of a shapeless material to a work piece or a supporting sheet. One can in principle distinguish between four groups of coating processes which each differ by the state of the coating material before coating:                1. Coating from a gaseous or vaporous state such as evaporation coating or metallizing;        2. Coating from a liquid, pulpy or paste-like state such as painting, dispersion or hot-melt coating;        3. Coating from an ionized state by electrolytic or chemical depositing such as among others the eloxal process or electrophoretic painting.        4. Coating from a solid i.e. granular or powder state such as powder coating or coating by sintering.        
Plastics have turned out to be the production material of choice for fields of application in which cheap single-use articles are used especially in medical diagnostics or environmental analysis. Since plastics and in particular cheap, easy-to-process commodity plastics that can be almost shaped at will, are usually composed of non-polar organic polymers, most of them cannot be wetted or only poorly wetted by polar media such as water and aqueous liquids, in particular biological samples. This property is utilized for certain applications especially where adherence of the liquid to the plastic object is not desired such as for disposable pipette tips. On the other hand there are numerous fields of applications for plastic objects in which a wettability with polar liquids is desired such as for example in the field of rapid diagnostics in which plastics can serve as materials for taking up biological sample liquids.
There is therefore no lack of different processes which have the aim of hydrophilically modifying plastic surfaces. Hydrophilic surfaces are characterized by a high surface tension which have a value near to the value of the surface tension of water (72 mN/m). Examples of a hydrophilic modification of plastic surfaces are corona plasma treatment, plasma chemical vapour deposition (PACVD, e.g. Antec Co. Kelkheim), the covalent binding of hydrophilic polymers with a photoreactive capability onto a plastic surface described for example in U.S. Pat. No. 4,973,493 (Photo Link Surface, BSI Corporation Co., Eden Prairie, Minn., USA), the application of layers containing wetting agents onto a plastic surface (for example Adhesive Research Co., Glen Rock, Pa., USA) or coating of inorganic-organic nanocomposites by means of sol-gel technology on the surfaces to be modified (for example INM, Co. Saarbrücken).
The processes mentioned above which each have advantages and disadvantages have been able to establish themselves for the hydrophilic modification of plastic surfaces whether of foils or formed pieces.
The corona plasma treatment leads to an increase in the surface tension which continuously decreases after the treatment over a short period of several days up to a few weeks. Moreover the surface tensions that can be achieved are relatively low.
Slightly higher surface tensions than those of the corona plasma treatment can be achieved with the PACVD technology. Although this method leads to surface coatings that are more stable over time, a decrease of the surface tension with time is still observed in this case.
Surfaces which are furnished with so-called photo link surfaces or which have been coated with layers containing wetting agents have a surface tension which is stable over time. However, both variants do not lead to optimally high surface tensions. An additional fundamental disadvantage of coating with layers containing wetting agents is that the wetting agents accumulate in a liquid aqueous sample that is contacted with the layer and can thus change it or make it unusable.
Coating of inorganic-organic nanocomposites by means of sol-gel technology on the surfaces to be modified leads to increased surface tensions. However, a disadvantage is that the process itself is very complicated and time-consuming which is why it does not appear to be suitable for modifying mass-produced plastic articles.
The object of the present invention was to eliminate the disadvantages of the prior art. In particular it was intended to provide surface coatings with a high surface tension that are stable over time to be used for example for the hydrophilic modification of objects. Moreover it should be possible to carry out the coating in a simple and reliable manner in order to be suitable for the modification of mass-produced articles.
The object is achieved by the subject matter of the invention as characterized in the patent claims.
The invention concerns a process for the production of a surface coating to increase the surface tension of objects characterized in that a layer of at least one element that can be oxidized with water or an alloy that can be oxidized with water is deposited on an object and this is at least superficially oxidized by the subsequent action of boiling water or water vapour on the deposited layer.
The invention additionally concerns the use of a surface coating to increase the surface tension of objects characterized in that the surface coating is obtained by deposition of a layer of at least one element that can be oxidized with water or an alloy that can be oxidized with water and subsequent action of boiling water or water vapour on the deposited layer.
Boiling water or water vapour and particularly preferably deionized water is preferably used to treat the deposited surface coating.